It is difficult to prove, but as many as 40% of all vehicle accidents could be due to tiredness. The risk of generating unfounded warnings which are therefore not accepted means that there is no way to safely warn drivers by the methods currently available.
Within the research community, it is accepted that there is a connection between tiredness and steering behaviour, how well a vehicle stays in its traffic lane and even how human beings blink. There is however no single threshold value or single function such that a certain value/behaviour would consistently indicate that a driver is tired.
U.S. Pat. No. 6,313,749 B1 relates to detection of vehicle driver tiredness, uses a number of different sensors to detect the vehicle's status and the driver's alertness, and converts the signals from the sensors to weighted factors which are used to adjust a model which reflects the driver's biological diurnal rhythm. The adjusted model is then used to generate warnings to the driver.
U.S. Pat. No. 6,661,345 B1 relates to a monitoring system for monitoring the alertness of a vehicle driver. The system comprises inter alia an acoustic sensor or a microwave sensor. The output signals from the sensor are processed using an algorithm with respect to driver alertness and a processed signal is generated. The processed signal is then used to assess the driver's alertness.
A difficulty with many of the systems currently used to warn the driver when he/she is tired is in the individual adaptation of systems, since it is not possible to use a general threshold value applicable to all drivers, which makes it inappropriate to warn the driver directly. For certain drivers, the systems will not generate any warning at all even if the driver is dangerously tired, whilst for other drivers the systems will generate a warning even if the driver is very alert. The systems therefore only marginally increase traffic safety and there is a risk that acceptance of all similar systems will be low if false warnings commonly occur.
Certain methods require a great deal of extra equipment which the driver must carry or information which the driver must put into the system/method manually.
Within the research community, it is accepted that there is a relatively general human diurnal rhythm. Researchers at Sweden's Stress Research Institute, Torbjörn Åkerstedt et al., have developed a model of the diurnal rhythm called the “Sleep/Wake Predictor” (SWP) model, which inter alia uses prior sleep and length of time since waking as a basis for approximating a person's alertness level.
More specifically, the model is based on three components, viz. alertness level (S), circadian rhythm (C) over a period of 24 hours, which characterises the biological sleep pattern, and 12-hour ultradian rhythm (U) based on diurnal biological activity, e.g. after a person has eaten, also known as the “after-lunch dip”.
The alertness level S is itself affected by three factors, namely the time of day, the length of time since waking and the duration of sleep. More specifically, S represents the time since waking and is modelled as an exponential function with a maximum value at the moment when a person wakes and an asymptotic decay at the end of the awake period. When the person goes to sleep, the way alertness is “recovered” during the sleeping period is that the alertness level rises quickly at the beginning of the sleep but the rate of increase declines asymptotically at the end of the sleep period.
The functions used to calculate S are as follows:S=L+(S(ta)−L)ed(t-ta)  (equation 1)where t is clock time in hours, ta the time when a person wakes, d the rate of decay and L the lower horizontal asymptote.S′=H−(H−S(ts))eg(t-ts)  (equation 2)where ts is the time when the person goes to sleep and H is the upper horizontal asymptote.
                    g        =                              1            8                    ⁢                      ln            ⁡                          (                                                H                  -                  14                                                  H                  -                  7.96                                            )                                                          (                  equation          ⁢                                          ⁢          3                )            
The constants in equations 1-3 have the default values L=2.4, d=0.0353 and H=14.3.
S′ represents the increase in tiredness of a person sleeping too little for a number of days in succession and caters for the difficulty of recovering over-quickly from a long period of shortage of sleep. This limitation has been introduced as a breakpoint to prevent too steep an increase in the exponential function for a specific value of S′.
The overall result is the following set of functions to determine S:
  S  =            {                                                  L              +                                                (                                                            S                      ⁡                                              (                                                  t                          a                                                )                                                              -                    L                                    )                                ⁢                                  ⅇ                                      -                                          d                      ⁡                                              (                                                                              -                            t                                                    -                                                      t                            a                                                                          )                                                                                                                                                                                    S                ⁡                                  (                                      t                    s                                    )                                            +                                                g                  ⁡                                      (                                          t                      -                                              t                        s                                                              )                                                  ⁢                                  (                                                            S                      b                                        -                    H                                    )                                                                                                        H              -                                                (                                      H                    -                                          S                      b                                                        )                                ⁢                                  ⅇ                                      g                    ⁡                                          (                                              t                        -                                                  t                          s                                                -                                                  t                          b                                                                    )                                                                                                              }        ⁢                            awake                                                                asleep              ⁢                              :                            ⁢                                                          ⁢              t                        ≤                          t              b                                                                                      asleep              ⁢                              :                            ⁢                                                          ⁢              t                        >                          t              b                                          
The constant Sb relates to the breakpoint and has the value 12.2 and the variable tb is the time when S is equal to Sb.
It should be noted that the invention is not limited to the constants stated above for determination of S, as other values of them may of course be used to adjust the calculations according to the prevailing circumstances.
The process C thus represents the body's biological clock, the circadian rhythm, and is modelled using a sine wave which has during the afternoon a maximum value defined as
  C  =            a      c        ⁢          cos      ⁡              (                              2            ⁢                                                  ⁢                          π              ⁡                              (                                  t                  -                                      p                    C                                                  )                                              24                )            where t is clock time in hours and the constants ac=2.5 and pC=18.
The process U represents the ultradian rhythm with a decrease in alertness at 15.00 hours which is defined as
  U  =            m      U        +                  a        U            ⁢              cos        ⁡                  (                                    2              ⁢                                                          ⁢                              π                ⁡                                  (                                      t                    -                                          p                      U                                                        )                                                      12                    )                    where t is clock time in hours and the constants mU=−0.5, aU=0.5 and pU=15.
S, C and U are calculated by putting a value for clock time t into the formulae.
Calculating S, C and U produces current values for them when t=0 and future values for them when t>0.
For a complete review of the SWP model, we cite
“Predicting road crashes from a mathematical model of alertness regulation—The Sleep/Wake Predictor.” Accident Analysis and Prevention, 40, pp. 1480-1485, by Åkerstedt, T et al. (2008). and
“Predictions from the three-process model of alertness”. Aviat. Space Environm Med, 75(3, Suppl.), A75-83, by Åkerstedt, T., Folkard, S., & Portin, C. (2004).
The SWP model thus makes it possible to determine the components S, C and U, and their aggregate can be used to calculate a value on a tiredness scale, the so-called “Karolinska Sleepiness Scale” (KSS), using the formulaKSS=10.9−0.6(S+C+U)
KSS may assume values of between 1 and 9, where low values mean that a person is alert and high values that a person is tired, for example:
1—very alert.
5—neither sleepy nor alert.
7—tired, but can stay awake without effort.
9—very tired, staying awake takes effort.
The object of the present invention is to propose an improved safety system for a vehicle which is easier to use than current systems and caters for the different activity and rest patterns of different drivers.